# Behaviors of \`deepcopy\` for a function constructed with different ways of referencing a global variable

**URL:** <https://discourse.julialang.org/t/behaviors-of-deepcopy-for-a-function-constructed-with-different-ways-of-referencing-a-global-variable/128939>\
**Category:** General Usage\
**Tags:** function, closure\
**Created:** [May 13, 2025, 12:23am UTC](https://discourse.julialang.org/t/behaviors-of-deepcopy-for-a-function-constructed-with-different-ways-of-referencing-a-global-variable/128939 "2025-05-13T00:23:01Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [May 13, 2025, 1:30am UTC](https://discourse.julialang.org/t/behaviors-of-deepcopy-for-a-function-constructed-with-different-ways-of-referencing-a-global-variable/128939/3 "2025-05-13T01:30:07Z")

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Thanks for the hint!! It seems like the way I thought of `Function`, as `struct` with attributes (fields) and methods, is no longer applicable in this case. Here’s what I understand now:

For `f1`, `box` was not an assigned field of the closure it returns, but simply a variable directly inside the definition (function body) of the closure, except it’s declared outside the closure scope. So, as soon as `f1` is defined, even before it is called, it is certain that there can only be one instance of the closure (which technically should not even be considered a “closure” based on your and [@danielwe](https://discourse.julialang.org/u/danielwe) 's comments ([1](https://discourse.julialang.org/t/is-it-possible-to-know-if-a-function-has-captured-variables/128934/9), [2](https://discourse.julialang.org/t/is-it-possible-to-know-if-a-function-has-captured-variables/128934/10)) on the other post) that `f1` generates. Thus, `typeof(f1c)` is deemed a singleton type. And because `typeof(f1c)` is a singleton, it should have one and only one instance (characterized by its type information), which is `f1c`. So `deepcopy` cannot just create another instance that is not identical to `f1c`. Therefore, `deepcopy` cannot sever the `f1c`’s reference to the global variable `box` either.

However, for `f2`, there can be many instances of the closure it generates, depending on the specific value of `localBbox` passed in. So `f2` in fact behaves like a struct with a single field (store the value of `localBbox` for each instance) and a corresponding method that uses the field. Hence, `f2c` is not an instance of a singleton type. Correspondingly, `deepcopy` also copies the data stored inside it.

Hopefully, what I described above is a more accurate mind model of how closures and functions with out-of-scope variables generally work. Thanks!!

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